Natural gas is desirable for use as a fuel for use to heat buildings, to supply heat for industrial processes, for the production of electricity, for use as a raw material for various synthesis processes to produce olefins, polymers and the like.
Natural gas is found in many areas which are remote from users of the natural gas. Since the natural gas is not readily transported as a gas, it is frequently liquefied for transportation in this more compact form.
A frequently occurring material in natural gas, which is also produced as a liquid when the natural gas is liquefied, is nitrogen. The nitrogen is also produced as a liquid but since it has a somewhat lower boiling point than liquefied natural gas (LNG), it frequently boils off after the liquefied gas is produced and stored. This can be a problem in that it takes a substantial period of time to remove a substantial amount of liquefied nitrogen from the bulk of the liquid comprising liquid natural gas and liquid nitrogen. Further the presence of the liquid nitrogen in the natural gas may result in difficulty in meeting specifications for the LNG. Accordingly, considerable effort has been devoted to the development of means for removing liquefied nitrogen present in LNG.
Various processes for the liquefaction of natural gas are known. Some such processes include U.S. Pat. No. 4,033,735 issued Jul. 5, 1977 to Leonard K. Swenson (Swenson) which is hereby incorporated in its entirety by reference. In such processes, a single mixed refrigerant is used. These processes typically use a mixture of gases to produce a single mixed refrigerant which can be compressed and liquefied to produce a refrigerant at a very low temperature, i.e., minus −230° F. to −275° F. or lower. The mixed refrigerant is passed into a heat exchanger and passed from a heat exchanger inlet, through the heat exchanger to an expansion valve at an outlet end of the heat exchanger and then directed back into the heat exchanger as vaporized and at the lower temperature. This stream is typically a continuously vaporizing stream as it passes back through the heat exchanger to the inlet end. The natural gas stream to be cooled is passed through the heat exchanger from its inlet end to its outlet in heat exchange with the vaporizing single mixed refrigerant. The spent refrigerant is then recovered, recompressed and re-expanded in the heat exchanger.
Another single mixed refrigerant process is shown in U.S. Pat. No. 5,657,643 issued Aug. 19, 1997 to Brian C. Price (Price) which is hereby incorporated in its entirety by reference.
Typically where the natural gas has contained substantial amounts of nitrogen; for instance, up to as high as 50 volume percent or more, then the liquid nitrogen is typically recovered with the liquid natural gas and allowed to boil off to the atmosphere or recovered for use. The LNG then, freed of a substantial portion of the nitrogen, is adjusted as necessary to have the desired properties for marketing as a fuel or other use.
A second type of process which has been used is illustrated by U.S. Pat. No. 3,855,810 issued Dec. 24, 1974 to Simon, et al (Simon) which is hereby incorporated in its entirety by reference. This patent shows a cascade type process. In such processes, a plurality of refrigeration zones in which refrigerants of decreasing boiling points are vaporized to produce a coolant, are used. In such systems, the highest boiling refrigerant, alone or with other refrigerants, is typically compressed, condensed and separated for cooling in a first refrigeration zone. The compressed, cooled highest boiling point refrigerant is then flashed to provide a cold refrigerant stream used to cool the compressed highest boiling point refrigerant in the first refrigeration zone. In the first refrigeration zone, some of the lower boiling refrigerants may also be cooled and subsequently condensed and passed to vaporization to function as a coolant for a second subsequent refrigeration zone and the like.
With either process, the produced LNG typically contains nitrogen in the LNG. The nitrogen is typically “flashed” off with methane from the LNG. The gas flashed off (flash gas) contains methane and nitrogen in widely varying proportions; however, methane is inevitably lost from the LNG. The flash gas may be used as a low BTU heating gas, passed to methane or nitrogen recovery, or both, or vented to the atmosphere. It would be desirable to produce the LNG with a no or very low nitrogen content.
A continuing effort to discover such a process has been directed toward this goal.